In a Code Division Multiple Access (CDMA) mobile communication system, an important way to raise the estimation accuracy of mobile station location is to identify whether the channel is a LOS channel or a NLOS channel, since the delay estimation error caused by a NLOS channel is the most serious one of those factors that affect the estimation accuracy of a mobile station location. The LOS channel can be understood as the transmitter is line of sight with the receiver, and the NLOS channel can be understood as the transmitter is non-line of sight with the receiver.
M. P. Wylie first proposes technique that identifies a NLOS used for mobile station location in the article named “The Non-Line of Sight Problem in Mobile Location Estimation” [1]. In Sep. 9th, 1997, he proposes a patent (with number: U.S. Pat. No. 5,974,329) to the USPTO named “Method and System for Mobile Location Estimation” [2], which aims at the NLOS error correction in the mobile location estimation and takes the NLOS identifying technique as a main element of the invention. In his invention the main thinking to identify a NLOS is as follow.
(1) Make long time recorders for distance between a UE (User Equipment) and a base station measured in every base station;
(2) Make smooth processing for the amount of recorded data;
(3) Identify a NLOS with the fact that the variance measured in a NLOS (after smooth processing, it is caused by geomorphology feature) is much greater than the variance measured in a LOS (caused by system measurement error).
The NLOS identifying method proposed by [1] and [2] need to use the time correlation properties of a mobile station, i.e., the method is suitable for identifying LOS channel of a mobile station in a moving state, but it is unsuitable for identifying a LOS channel of a mobile station in a static state; furthermore, the method need to accumulate data for a longer period of time, which will cause a longer delay and is difficult to satisfy response time requirement of the FCC.
A patent (number CN 01105808.0) named “Method and Device for Identifying a NLOS channel in a CDMA cellular communication system” [3] has been applied by the applicant; in this patent, a LOS identifying method is proposed, which synthetically uses channel loss and multipath power profile to identify a LOS channel.
Although the method in [3] has overcome the disadvantage of the method in [1] and [2] which only identify the LOS at the moving state of a mobile station, but in [3], the method has not used the power difference between paths, and, the ratio of the mean delay and delay spread {overscore (τ)}/σ in a same power delay profile to identify a LOS, also the selected characteristic parameters are not simple and clear enough, so the implementation is relatively complex.
In the article: “ Characterization of UHF multipath radio channels in factory buildings” [4] written by T. S. Rappaport, IEEE transaction on antenna and propagation, vol.37, no.8, pp.1058–1069, August, 1989, based on the indoor measured data, {overscore (τ)}/σ difference between LOS channel and NLOS channel at an indoor environment has been discussed, i.e., regularity of multipath power profile in LOS channel and NLOS channel at the indoor environment has been discussed; the conclusion is that less {overscore (τ)}/σ shows that energy is concentrated on the first path and larger {overscore (τ)}/σ shows that energy is concentrated on the middle and trail part of the power delay profile. In reference [4], the discussion about {overscore (τ)}/σ is based on that the LOS distance is known and thereby the arriving time of LOS path can be computed; but in real the detected first path may not be the LOS path and also how to compute and use regularity of {overscore (τ)}/σ has not been discussed there.